2. Field of the Invention
The present invention relates to a method and apparatus for producing a high-purity titanium. The present invention also relates to a high-purity titanium target for sputtering. Particularly, the present invention relates to a production of a high-purity titanium which has a purity appropriate for producing a semiconductor device by using the titanium target.
2. Description of Related Arts
Along with a high integration of LSIs, attention has been focused on the high melting-point metals, such as molybdenum, titanium and tungsten.
Particularly, in the case of titanium, which is one of the high melting-point metals, its utilization as metallic Ti, TiSi, TiN and the like has attracted interest ("VLSI Production Technique" (in Japanese) Nikkei BP Co., Ltd. Published on Jan. 14, 1989, page 167). Particularly, TiSi can be used as a barrier metal of an Si-gate NOS transistor, which is a subject of interest. In addition, there are attempts to substitute titanium for aluminum, heretofore used as conductors of LSIs and NOS electrodes. Research is advanced particularly in the use of titanium as a conductor having a fine pattern. A representative method for forming the barrier metals, conductors and the like by using titanium, is sputtering, in which a target made of titanium is sputtered in an argon atmosphere.
The titanium target is produced by shaping, sintering, and melting commercially available titanium, followed by machining.
In order to enhance the performance-reliability of the components of a semiconductor device, such impurities as follows must be decreased.
(1) Alkali metals such as Na, K, Li and the like
(2) Radio active elements such as U, Th and the like
(3) Heavy metals such as Fe, Cr and the like
(4) Oxygen
The alkali metals, such as Na and K, move easily in the gate-insulation layer of MOS transistor and become a cause of deterioration in the properties of interface between the insulation layer and Si. Radio-active elements such as U emit rays therefrom, thereby causing the soft error of LSI memory. Heavy metals such as Fe also cause trouble at the bonding interface. Oxygen incurs deterioration of properties of MOS transistor.
The purity of a titanium target used for producing recent 1MDRAMs and 4MDRAMs is 5N (99.999%) except for the gaseous components. More specifically, the content of alkali metals, must be 0.1 ppm or less for each of Na, K and the like. The content of radio active elements must be 1 ppb or less for each of U, Th, and the like. The content of heavy metals must be 0.5 ppm or less for each of Fe, Cr and the like. The oxygen content must be 150 ppm or less, preferably 100 ppm or less.
Pure titanium, which is industrially produced at present, contains a large amount of the above mentioned alkali metals, heavy metals and gaseous components. Pure titanium having such a poor level of purity cannot be used in the field of semiconductors.
Japanese Unexamined Patent Publications Nos. 62-294177 and 62-294179 describe a thermal decomposition method of titanium iodide, for further purifying industrially produced titanium. This method, however, involves limitations in the refined purity. That is, the Fe content in the examples of these publications is 50 ppm, and hence is exceedingly higher than the required level.
The above method is, therefore, not suitable for producing the semiconductors. In addition, the thermal decomposition method is inherently adapted for a laboratory scale and is not suitable for industrial production. This is because the titanium-deposition rate is too small, for example, 0.225 g of Ti per hour.cm.sup.2. Provided that the diameter of a titanium plate is 10 cm, 176 g of titanium is obtained in 10 hours, i.e., only 17.6 g of titanium is obtained per hour. In the thermal decomposition method of titanium iodide, induction heating is used to maintain the decomposition at a high temperature, i.e., 1100.degree. to 1500.degree. C. The deposition amount of titanium per electric power is 0.59 g/kwh, a very small amount. As is described above, the thermal decomposition method of titanium iodide involves limitations in purity, and low productivity, and, is further, extremely expensive.
Another known method is the fused-salt electrolytic method. The fused-salt electrolytic method has been studied up to now for electrolytic extraction of titanium from difficult to reduce titanium ore. Consideration was also made as to how to refine sponge titanium by means of the above method. For example, in "Bureau of Mines" (1957, pages 1-43) the following method is described (c.f. "Report of Investigation"). The mixed salt of LiCl-KCl is admitted in a crucible made of iron and is fused. The TiCl.sub.4 is first blown onto the Ti strips which are put in the molten salt. An iron basket, in which the sponge Ti is contained, and a Ti cathode, are admitted in the fused salt. The electrolysis between the Ti sponge as the anode and the Ti cathode is carried out while using TiCl.sub.2 as the carrier, thereby depositing pure titanium on the Ti cathode. The Fe content of pure titanium deposited by the above described method is 200 ppm and hence is very high. Electrolysis, which is similar to the one described above, is carried out in the other apparatus using a cell made of soft steel. The Fe content of pure titanium deposited in this apparatus is 110 ppm. It is impossible in any one of the apparatuses to attain the refinement at the level of Fe&lt;1 ppm, since the iron content obtained by the above methods is extremely high.
The present inventors instead considered an iron crucible or container made of such materials as stainless steel or graphite, which can withstand the fused salt containing TiCl.sub.4 and TiCl.sub.2. Any one of such materials is, however, subjected erosion caused by the fused salt when it reaches a high temperature of from 500.degree. to 850.degree. C. and is partially dissolved, thereby causing the contamination of deposited titanium. The refined products contain oxygen at a high content, as well.